This invention generally relates to gradient amplifier systems and more particularly to a system for managing the movement of energy within a high voltage gradient amplifier system for powering the gradient coils of a magnetic resonance imaging (MRI) apparatus.
The demands of the MRI market have continuously required faster gradient fields. These spatially varying magnetic fields are produced by large coils, which are driven by gradient waveforms produced by a gradient amplifier system (GAS). The GAS is capable of high current and high voltage. The high current enables greater Gauss per centimeter gradient fields, while the high voltage enables faster gradient field slew rates.
Over the past decade, a number of imaging sequences have been developed which require unconventional gradient waveforms. The conventional waveforms have been primarily trapezoidal in nature, with fast linear ramps followed by extended plateaus. These waveforms require a large peak voltage to average voltage magnitude ratio, and can therefore be supplied by a GAS which inefficiently produces large output voltages, but efficiently produces moderate output voltages.
Some of the more recently developed imaging techniques, such as spiral scanning and diffusion weighted imaging, require not only high peak currents, high RMS currents, and high duty cycle, but also continuous slewing of the gradient field. Consequently, a GAS is required which can produce large peak and RMS voltages along with large peak and RMS currents. The increased demand for high RMS and peak voltage gives rise to the need for a GAS design with improved efficiency.
One way to address the increased peak voltage demands of the recently developed imaging techniques is to design a GAS with amplifiers connected in series, each having an expensive power supply. Another technique is to employ booster amplifiers and reclaim a portion of the power according to techniques commonly known in the art.
However, present generation booster amplifiers are too inefficient to operate continuously in a switching mode. Thus, these conventional booster amplifiers can be regenerated or re-charged only at the beginning and the end of a current pulse. Special pulses may be provided in the imaging sequence solely for the purpose of enabling this regeneration. This, however, increases the cost of programming and operating the GAS.
The present invention provides a high voltage GAS for powering gradient coils in an MRI system. The GAS is capable of economically and efficiently providing current slew rates of arbitrary form and duration by controlling the operation of series connected amplifier modules and managing regeneration of the amplifier modules depending upon the characteristics of the output waveforms.
Each amplifier module provides only a portion of the system voltage output, but is rated at the overall system current. The amplifier modules may themselves be made up of a plurality of paralleled amplifier stages. By appropriately altering the output voltage and current of each of the series connected amplifier modules, the power required by all but one of the amplifier modules may be reduced to zero during times of non-quiescent load current. During quiescent loading, small bias supplies are sufficient to support the internal quiescent losses of these floating amplifier modules (FAMs).
The small bias supplies may be either isolated, or boot strapped. Either implementation yields a significant reduction in power supply complexity and cost. Only one amplifier module (the ground-referenced-amplifier module (GAM)) requires a large power supply, which may be a split rail, ground-referenced, non-isolated power supply. In addition, this power supply does not need to be highly regulated. In fact, this supply can be little more than a rectifier with current interrupting capability, thereby further reducing the cost of the power supply.
In an MRI apparatus, three gradient coils are typically present, one for each of the spatial axes. The three GAMs corresponding to the three axes can therefore all be powered by the same non-isolated power supply. The end result is a three axes GAS that is economical, yet has high voltage and arbitrary gradient current slew rate capability.
The FAMs of each axis need only small power supplies because they receive power from the GAM. Since all of the amplifier modules conduct the same current, power can be transferred from the GAM to the FAMs by increasing the GAM""s output voltage and decreasing the voltage across the FAMs by an equal amount when positive currents flow to the load. The voltage polarities are reversed for negative load currents. Since the GAM voltage and the FAM voltages are equal in magnitude and opposite in polarity, they cancel at the load. Accordingly, very fast error corrections can be made without inducing distortion into the load loop.
The voltage that must appear at the load depends upon the gradient command signal, which in turns depends upon the desired gradient waveform. Thus, the load voltage is a dependent variable. Consequently, the voltage alterations at the output of the GAM may reduce the summed voltage alterations of the FAMs. In addition, the controller that alters the output voltage of the FAMs and the GAM, the Energy Management Controller (EMC), will ultimately produce current dependent degeneracy and current dependent regeneracy, respectively, because power must flow from the GAM to the FAMs where it is dissipated.
One embodiment of the EMC results in a zero-sum perturbation to the load, meaning that as the voltage is increased in one amplifier module, it is decreased by the same amount in another module such that the load never perceives a voltage change. With a zero-sum configuration, the EMC can produce signals which are non-linear and non-stationary, greatly enhancing the flexibility to the EMC.
In general, the EMC includes a regulator function which receives a feedback signal indicating the state of the energy or energy flow in the FAMs. It also includes inputs for a current and/or a voltage related to the gradient current, and the gradient voltage or amplifier module output voltage. The EMC can process these signals in a variety of ways, including non-linear functions such as multipliers, squarers, absolute value functions, and saturating gain block functions (e.g., a sgn ( ) function). Additionally, by properly phasing the PWM drive of each of the FAMs, the effective load ripple frequency can be increased and its amplitude reduced.
These and other features will become more apparent and the present invention will be better understood upon consideration of the following description and the accompanying drawings.